The present invention relates to an electrophotographic photoreceptor having a photoconductive layer made of an amorphous silicon series material.
Recently, attention has been paid to amorphous silicon hydride (a-Si(H)) as a photoconductive material for electrophotographic photoreceptors, and the development of such amorphous silicon hydride material is being promoted, since amorphous silicon hydride has excellent light-sensitivity and heat-resistance and has a high hardness. Additionally, a thin film of a-Si(H) having a large area can be obtained relatively easily, and equally important is the fact that this does not cause environmental pollution.
A layer of a-Si(H) is generally formed from a silicon-containing gas, such as silane (SiH.sub.4) gas, as a raw material gas by the plasma controlled vapor deposition (CVD) method. The a-Si(H) layer formed by that method has a low localized level in the forbidden band and has a high photoconductivity. In addition, when a sufficient quantity of diborane (B.sub.2 H.sub.6), phosphine (PH.sub.3) or other appropriate gas is incorporated into the raw material gas, electroconductivity control and valence electron control are possible and increased resistance is also possible. Moreover, when an appropriate gas is incorporated into the raw material gas, carbon (C), nitrogen (N), oxygen (0) and the like can be introduced into the resulting a-Si(H) material, so that necessary characteristics, such as charge acceptance, photoconductivity, temperature characteristics, mechanical strength, etc., which are required for the photoreceptor material, can be imparted to the a-Si(H) layer. Accordingly, a photoreceptor having an a-Si(H)-containing light-sensitive layer has extremely desirable properties.
However, although a photoreceptor having a surface layer of such a-Si(H) can produce good images when it is new, it has been known that a photoreceptor of this kind often yields defective images when it is used for image duplication after being exposed to air or to a high moisture atmosphere for a long period of time. In addition, it has also been known that images formed with such a photoreceptor are often of poor quality after the photoreceptor has been used for a large number of repeated duplications. Further, it has been confirmed that degradation of the photoreceptor often causes loss of image quality particularly in a high moisture atmosphere, and the critical humidity level which results in low image quality gradually decreases with an increase in the number of duplications made with the photoreceptor.
As mentioned above, it is believed that in a photoreceptor having a surface layer of a-Si(H) the outermost surface of the photoreceptor is often affected by exposure to air or moisture for a long period of time or by chemical agents (such as ozone, nitrogen oxides, nascent state oxygen, etc.) formed by the corona discharge in the duplication process, whereby defective images are produced as a result of some chemical modification of the photoreceptor. However, the mechanism of such photoreceptor degradation has not yet been ascertained sufficiently.
In order to prevent the generation of such poor quality images and to improve the image quality, durability and the moisture-resistance of the photoreceptor, a means of providing a protective layer on the surface of the photoreceptor for chemical stabilization has been proposed and tried.
For instance, the published unexamined Japanese Patent Application (OPI) No. 115559/82 discloses a method of providing hydrogenated amorphous silicon carbide (a-Si.sub.x C.sub.l-x (H), 0&lt;x&lt;1) or hydrogenated amorphous silicon nitride (a-Si.sub.x N.sub.l-x (H), 0&lt;x&lt;1) as a surface protective layer so as to prevent the deterioration of the surface layer of the photoreceptor by the duplication process or the environmental atmosphere. However, although the image quality durability can fairly be improved merely by a controlled choice of the carbon concentration or the nitrogen concentration in the surface protective layer, the photoreceptor moisture-resistance cannot be maintained in a high humidity atmosphere (e.g., having a relative humidity of 80% or more), and therefore, the image quality degradation occurs at about 60% relative humidity after the photoreceptor has been used to make several tens of thousands of copies. Thus, in the present situation, image quality durability and the photoreceptor moisture-resistance cannot be improved significantly even by the provision of the surface protective layer of the kind described in this Japanese OPI.
More recently, it has become known that an amorphous carbon (a-C) is extremely effective as a material for such a surface protective layer. However, although the chemical stability and the moisture-resistance of the photoreceptor are greatly improved by the provision of an a-C layer, there still is a problem with the image quality-durability of the photoreceptor, since the mechanical strength of the a-C material used in the protective layer is insufficient to permit a developing agent to be applied to the surface of the photoreceptor for image formation or to resist mechanical loads, such as from a cleaning blade to be applied thereto.
The object of the present invention is to overcome the above-mentioned defects and to provide photoreceptors which have excellent moisture-resistance and image quality durability and are free from deterioration in those characteristics even after storage for a long period of time or after repeated use and are also almost free from deterioration of other characteristics including power decrease or image degradation even after use in a high humidity atmosphere, and which additionally have a surface which is highly resistant to abrasion or damage resulting from the actual image formation process, including development and cleaning.